Modified zipper elements for easy open containers
BACKGROUND OF THE INVENTION 
     The present invention relates to improvements in flexible containers, and in particular, to modifications of the zipper elements for an easy open container. 
     Recloseable, flexible containers are well-known in the art. Such containers normally comprise a bag-like structure made from a folded web of thermoplastic film, and typically include interlocking male and female zipper elements fused, extruded or attached to the bag side walls. The bag-like structure is created when the thermoplastic film is folded, severed and sealed along its side edges. The zipper elements are positioned on the opposing bag side walls and close to seal the bag mouth. 
     Because recloseable, flexible containers are, by definition, accessible, positive means to completely seal the bag mouth have been developed to preserve freshness, prevent accidental opening, and inhibit intentional tampering. In turn, means to open such sealed containers have also been developed. For example, tear strips are known which comprise weakened lines for tearing across sealed webs to gain access to the mouth of the bag. Naito, U.S. Pat. No. 3,198,228 and Uramoto, U.S. Pat. No. 3,780,781 (the &#39;781 patent) both show tear strips having such weakened regions. 
     Problems have arisen, however, in the use of containers having weakened regions for tear lines. Such problems include consumers&#39; inability to tear the thermoplastic material; the need, despite the weakened regions, to use scissors to open the container, obviating the value of the tear strip; and irregular tearing down the bag lips, which compromises the strength, shape and operation of the zipper elements. Modifications, such as the inclusion of perforations along the weakened lines of tearing, have enhanced the efficacy of tear strips, but eliminate the airtight and tamper-proof features desired in completely sealing the mouth in such containers. 
     Other tear strips comprised of a thin thread or string of fiber placed within the container at its top are shown by Staller, U.S. Pat. No. 3,543,343, as a means to open sealed containers along a single tear line. The drawback of such tear strips is that the tear line may deviate from the top of the bag and propagate into the zipper elements, again compromising their strength, shape and operation. In an apparent effort to avoid such problems, Uramoto, in U.S. Pat. No. 3,827,427 (the &#39;427 patent), defines two tear lines, one on each lip of the bag mouth, between pairs of ribs. Further development is needed, however, to improve the design of tear strips for the easy opening of sealed flexible containers of thermoplastic material. 
     SUMMARY OF THE INVENTION 
     The present invention meets that need by providing an improved means for easy opening of sealed flexible containers. In the present invention, a tear strip comprised of a bead of thermoplastic material is provided to define a tear line generally along the top edge of the flexible container. The tear strip may be extruded into the thermoplastic film or extruded separately and then attached to the surface thereof. The irregular tearing of flexible containers which has been experienced with tear strips in prior art devices is limited in the present invention by extending the bases of the zipper elements upward towards the top of the flexible container along both of the bag lips. Because of the length and additional reinforcement provided by the extended bases, a tear line propagating and deviating down the bag lips will stop when encountering the extended bases and will continue generally in the intended direction of tearing across the mouth to open the container without compromising, tearing, or distorting the interlocking zipper elements. 
     The extended bases of the present invention do not comprise weakened regions which define the primary tear lines for opening the bag. Rather, the extended bases provide safe boundaries to contain irregular tearing to facilitate the use of a tear strip along the top edge of the container. As well the side walls or bag lips remain intact and are not selectively weakened. 
     The present invention solves the tearing problems encountered with prior art devices without requiring multiple elements or ribs. Instead, a simple one-piece extended base is provided in the present invention which both limits the tear line and supports the zipper elements. The extended bases are attached to the thermoplastic film, and the zipper elements are attached to the extended bases, by any of the full range of methods available in the art, such as fusion, welding, bonding, extrusion or adhesion. 
     Once the container is opened, the extended bases of the zipper elements described above may function as grip strips and may be embossed, or include ribs, ridges, embedded particles, or similar gripping surfaces. Other features may be added to the container design such as sealed notches near the top edge of the flexible container, but below the tear strip, to facilitate initial propagation of the tear line. Similarly a pattern of one or more sealed apertures may be placed below the tear line to encourage propagation of the line adjacent to the tear strip without compromising the airtight nature of the container. 
     A method for making the present invention is also provided wherein a source of thermoplastic film is provided; extended bases are extruded, spaced apart and fused thereto; zipper elements and a tear strip are extruded; the zipper elements fused to the extended bases; and the tear strip fused to the thermoplastic film in a space between the extended bases. Thereafter the film is folded severed and sealed to provide an easy-open flexible container. 
     Accordingly, it is an object of the invention to provide an easy open flexible container wherein a simple tear strip may be used without Compromising the interlocking zipper elements of the flexible container. It is a further object of the invention to provide a means for making such an easy-open flexible container. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a cross-sectional view of thermoplastic film as used in the prior art to make sealed flexible containers. 
     FIG. 2 is a cross-sectional view of the preferred thermoplastic film of the present invention. 
     FIG. 3 is an easy-open, sealed flexible container made of the thermoplastic film shown in FIG. 2. 
     FIG. 4A is an enlarged cross-sectional view of the preferred embodiment of FIG. 3 taken along line 4--4. 
     FIG. 4B is an enlarged cross-sectional view of an alternative embodiment of the present invention, taken along line 4--4 of FIG. 3. 
     FIG. 5 shows the various ways to use the tear strip in the preferred embodiment to open the flexible container. 
     FIG. 6 is a sealed flexible container showing sealed apertures to facilitate the use of the tear strip. 
     FIG. 7 is an enlarged cross-sectional view of the embodiment of FIG. 6 taken along line 7--7. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows thermoplastic film 10 from which containers are made in the prior art. FIG. 2 shows the improved design of the present invention. When the thermoplastic sheet material of FIG. 2 is folded, severed and sealed, the flexible container 12 of the present invention results, as shown in FIG. 3. As seen in FIG. 3, container 12 has sealed edges 14, 16, and 18. Upper edge 20 is formed by folding, and tear strip 22 is located therein as further shown in FIG. 4A. Preferably, sealed notches 24 shown in FIG. 3 are included to aid consumers in opening container 12 with tear strip 22. Notches 24 are sealed around their edges to maintain the airtight integrity of container 12. Once opened, the remaining bag-like portion of container 12 may be reclosed and resealed by mean of interlocking zipper elements 26 and 28 located on side walls 30 and 32. Extended bases 36 and 38 shown in FIGS. 3 and 4A, reinforce thermoplastic film 10, and serve to limit the tearing of thermoplastic film 10 by tear strip 22 to the lip areas 34 of side walls 30 and 32 above bases 36 and 38. Extended bases 36 and 38 thereby protect interlocking zipper elements 26 and 28 from damage upon opening container 12. Extended bases 36 and 38 may, alternately be termed base layers, reinforcing strips or even enlarged grip strips, but, having the characteristics and advantages described, are herein referred to as extended bases. 
     In the preferred embodiment of FIG. 4A, the thermoplastic film 10 of FIG. 2 is preferably a multi-layer sheet comprising an outer barrier layer 8 of two to five mils thick polypropylene, polyester or Saran (polyvinylidene chloride), and a sealant layer 6 of one to three mil thick polyolefin or adhesive. The extended bases 36 and 38, zipper elements, 26 and 28, and tear strip 22 are preferably made of a low density polyethylene. As shown in FIG. 4A, the tear strip 22 may be formed as a bead of thermoplastic material fused, welded, bonded, adhered, or attached by other known means, onto the inside of upper edge 20 of container 12. Preferably extruded and fused onto the surface of film 10, tear strip 22 is of sufficient thickness to be stronger than the thermoplastic film 10. 
     As also shown in FIG. 4A, extended bases 36 and 38 may similarly be fused, welded, bonded, adhered, extruded or attached by other known means to the inside of side walls of 30 and 32. Preferably, extended bases 36 and 38 are separate extrusions two to three mils thick which are fused onto side walls 30 and 32, and zipper elements 26 and 28 are separate extrusions which are then fused onto extended bases 36 or 38. As such, the construction of the preferred embodiment involves a simple manufacturing process relative to more complex prior art devices. Alternatively, extended bases 36 and 38 and zipper elements 26 and 28 may be an integral extrusion of extended bases and zipper together, which integral extrusion may then be fused onto thermoplastic film 10. 
     The alternative embodiment of FIG. 4B shows an embodiment wherein tear strip 22 is, representatively, shown outside of container 12. Tear strip 22 could also function with the other elements shown if located inside of container 12. Extended bases 36 and 38 and zipper elements 26 and 28 are, as before, separate extrusions. However, as shown in FIG. 4B, zipper elements 26 and 28 include short bases which short bases are later fused, welded or similarly attached by known means to extended bases 36 and 38. 
     As seen in FIG. 5, the present invention permits tear strip 22 to be pulled through generally the entire range of motion--up, generally down, and sideways--to open container 12. Any tendency for tear line 22&#39; to propagate or deviate down lips 34 is inhibited by bases 36 and 38, avoiding injury to zipper elements 26 and 28. Moreover, extended bases 36 and 38 sufficiently reinforce lips 34 and side walls 30 and 32 that lateral tearing forces result in a clean tear across the mouth 40 of flexible container 12. Once opened, extended bases 36 and 38 may function as grip strips, and may be embossed, or include ribs, extruded ridges, embedded particles or similar means to provide gripping surfaces. Finally, as shown in the alternative embodiment of FIGS. 6 and 7, additional sealed apertures 42 intermediate between edge seals 14 and 18 may be provided to further guide tear line 22&#39; (not shown) across mouth 40. The sealed edges 44 of apertures 42 maintain the airtight integrity of container 12 while defining an excised area in the lips 34. 
     In a further aspect of the present invention, a method is provided for producing an improved, easy-open sealed flexible container 12. The preferred method comprises supplying a source of thermoplastic film 10, extruding the two extended bases 36 and 38 onto thermoplastic film 10 at a distance apart; attaching extended bases 36 and 38 thereto, preferably by applying pressure with a pair of rollers; thereafter extruding a means for tearing, such as a bead of thermoplastic material, onto thermoplastic film 10 in the space between the extended bases 36 and 38 and attaching the means for tearing 22 onto thermoplastic film 10; extruding at least one pair of interlocking zipper elements 26 and 28, and attaching mating ones of those zipper elements onto separate extended bases, and separating those zipper elements as far apart as practicable; folding thermoplastic film 10 generally along the line defined by said means for tearing 22 so as to bring into communication interlocking zipper elements 26 and 28; and thereafter sealing and severing the thermoplastic film 10 to define a sealed flexible container 12. 
     The method may further comprise the step of severing and sealing at least one notch 24 on at least one edge of flexible container 12 in the lip area 34 below the means for tearing 22 and above the extended bases 36 and 38. The method may also further include cutting sealed apertures 42 and the lip area 34 below the means for tearing and above the extended bases 36 and 38. In addition, the method may include the step of extruding means for gripping onto the extended bases 36 and 38, such means for gripping being ribs or ridges. As well, means for gripping may be applied to the extended bases such as by embossing a pattern on the surface of extended bases 36 and 38 or embedding particles therein. 
     The precise order in which extended bases 36 and 38 and means for tearing 22 are extruded and attached to thermoplastic film 10 is not critical, so that the means for tearing 22 may first be applied to thermoplastic film 10 after which extended bases 36 and 38 may be applied either side of means for tearing 22. In the preferred embodiment, extended bases 36 and 38 are first applied to thermoplastic film 10 after which zipper elements 26 and 28 are attached to extended bases 36 and 38. However, alternatively, extended bases 36 and 38 could have zipper elements 26 and 28 first applied thereto, after which extended bases 36 and 38 could be attached to the thermoplastic film 10. In this regard, zipper elements 26 and 28 could also, alternatively, be extruded with short bases which thereafter may be attached to extended bases 36 and 38 to result in the flexible container 12 shown in FIG. 4B. 
     In an alternative method, extended bases 36 and 38 and zipper elements 26 and 28 are an integral extrusion. In this alternative method, as well, integrally extruded bases 36 and 38 and zipper elements 26 and 28 may be attached to thermoplastic film 10 before or after means for tearing 22 is extruded and attached to thermoplastic film 10. The addition of means for gripping to extended bases 36 and 38 is also possible in the alternative method in similar fashion as described above. 
     While certain representative embodiments and details have been shown and described for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the device and method disclosed herein may be made without departing from the scope of the invention which is defined in the appended claims.